The present invention relates generally to high definition color television (HDTV), and, more particularly, to HDTV that is compatible with existing television receivers using the format developed by the National Television Systems Committee (NTSC).
Current television receivers, referred to as xe2x80x9cNTSCxe2x80x9d for National Television Systems Committee, are analog devices originally developed in the 1930""s for transmission of black and white images. The system uses interlacing of lines in frames, a complex arrangement for synchronization of the horizontal and vertical retraces, and allocates a portion of the signal voltage range for blanking during retrace. When color was incorporated in the 1950""s, sophisticated subcarrier modulation techniques were added, along with accommodations to the human visual response that allowed the new chrominance data to fit within the earlier format. Although simple in principle, NTSC television is actually very detailed in practice, and represents a generally under-appreciated triumph of electronics technology. (Appendix A contains a very brief summary of the pertinent NTSC parameters, and Reference 1 gives extensive details.)
In HDTV, a picture is converted into digital data and compressed, like files on a computer. One part of the compression process also is used under the NTSC format where the data is split into luminance data (black and white fine detail information) and two components of chrominance (color) data, which require far less detail. In the HDTV system, the picture is further transformed into another xe2x80x9cspacexe2x80x9d where the files can be compressed very efficiently. At this time, the compression system is the Joint Photographics Experts Group (JPEG) system (or its moving picture variation, MPEG), which is used in conventional computers for picture compression.
HDTV systems are modern digital computers that use information theoretic methods to transmit and receive the digitized image data efficiently (see Reference 2 for the Discrete Cosine Transform (DCT)-based standard, References 3 and 4 for information-theoretic methods). DCT transformations also exploit features of the human visual response to minimize the number of bits required for transmission of color images, which are perceived with nearly the same quality as the original image. The visual features that are used include persistence of vision, spatial resolution of different color components, and the contrast threshold limits needed to distinguish brightness variations at different spatial wavelengths.
The last of these features naturally leads to a transformation of blocks of pixels from brightness values to coefficients of basis functions of some appropriate form, such as DCT, other trigonometric functions, or wavelets. These coefficients are then xe2x80x9cquantizedxe2x80x9d, or grouped into bins spanning a range of values, so that the entropy of the resulting set of coefficients is small enough for the information to be accommodated by the channel. This quantization step is noninvertible, or lossy, but generally provides the largest single contribution to the image compression. Further image compression is obtained by using entropy coding to reduce the number of bits to be transmitted, nearly reaching the fundamental limits on the channel capacity as determined by bandwidth and noise (Reference 3). The most efficiently encoded signals reach the highest entropy rate attainable; that is, they resemble gaussian random noise. The HDTV receiver samples the waveform digitally, corrects for errors, inverts the entropy coding and mathematical transformations, and displays the resulting image. If such signals were presented to an NTSC television receiver, they would appear as xe2x80x9csnowxe2x80x9d. (Reference 5 gives an excellent overview of NTSC and HDTV systems.)
During the interim period when existing NTSC receivers are still abundant, some method is needed to allow new programming in HDTV format to be seen also on NTSC receivers. Digital-to-NTSC converters were proposed at one time, and the current approach is the simultaneous broadcasting of an additional channel that contains the NTSC signal. It is desirable, however to minimize the amount of bandwidth that must be utilized for HDTV. It is a purpose of the present invention to provide HDTV transmission within a single 6 MHz channel and in a format that is readily usable by either a NTSC receiver or a HDTV receiver.
As more fully explained below, the present invention uses an alternate scheme to compress the luminance and chrominance data. Only high resolution details are compressed digitally, as described above for HDTVs; the remaining low resolution details are transmitted in the NTSC format. NTSC receivers lose very little picture quality since the finer details are not discernible to the human eye in an analog TV. To be fully compatible with the NTSC format, additional xe2x80x9cbookkeepingxe2x80x9d information is added, such as a color burst to synchronize the reception of the color data and the synchronization pulses that trigger line sweeps. There is also some xe2x80x9cdead timexe2x80x9d when the electron beam is moving back to start a new line. However, there is increased compression when digital data is separated into two or more components, which partially offsets these channel capacity losses. The result is that a single channel can be used to transmit the combined signal with little loss in HDTV picture detail.
In an NTSC receiver, in the exemplary process herein, about half the extra digital information containing the finer details are seen in xe2x80x9cletterboxxe2x80x9d bounding lines that may be displayed above and below the wider format picture produced under the HDTV format. In the examples presented here, the letterbox lines are not black, but rather a rather uniform gray since they are replaced every 60th of a second. The other half is sent in the vestigial video sideband that is not currently used by NTSC transmitters. However, conventional NTSC transmitters can be readily modified to use the vestigial sideband. The NTSC receivers will not use this data at all since it is sent in quadrature with the carrier wave for the video signal and is not processed by simple amplitude modulation detectors.
The new HDTV receivers can be readily modified to accept the combined signal since only a software change is needed to accept the combined data. The low resolution data sent in NTSC format is converted to digital form and combined with the finer detail data to form the complete image that is displayed by the HDTV receiver.
Various advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The present invention includes a method for HDTV and NTSC compatible image processing. Luminance and chrominance image data of a scene to be transmitted is obtained. The image data is quantized and digitally encoded to form digital image data in HDTV transmission format having low-resolution terms and high-resolution terms. The low-resolution digital image data terms are transformed to a voltage signal corresponding to NTSC color subcarrier modulation with added retrace blanking and color bursts to form a NTSC video signal. The NTSC video signal and the high-resolution digital image data terms are then transmitted in a composite NTSC video transmission. In a NTSC receiver, the NTSC video signal is processed directly to display the scene. In a HDTV receiver, the NTSC video signal is processed to invert the color subcarrier modulation to recover the low-resolution terms, where the recovered low-resolution terms are combined with the high-resolution terms to reconstruct the scene in a high definition format.